This invention relates to DC circuit interrupting apparatus suitable for use in high voltage large capacity DC transmission systems.
In an AC circuit interruptor, electric are struck between separated contacts extinguishes when the current passes through a zero point. However, in the case of a DC circuit interruptor, since there is no zero point in the current and voltage it is necessary to forcibly reduce the current to zero and many devices have been proposed for this purpose.
FIG. 1 shows one example of such device in which a circuit interruptor CB in the form of a vacuum switch or a circuit breaker utilizing an arc extinguishing medium such as SF.sub.6, air, oil etc. is connected in series with a DC power line and an oscillating circuit including a capacitor C, a reactor L and a switch S is connected across the circuit interruptor CB. When the circuit breaker CB is opened, switch S is closed to pass the oscillatory discharge current of capacitor C through the electric arc struck between the contacts of the circuit breaker CB thereby forming a zero point in the current to be interrupted.
FIG. 2 shows a portion of a DC transmission system utilizing the circuit interrupting apparatus shown in FIG. 1 in which Tr represents a transformer, Re a rectifier or inverter, Ls a smoothing reactor and F a filter including a capacitor. When the DC current I is interrupted by the high frequency oscillatory discharge current, an energy expressed by 1/2 LS I.sup.2 would be stored in the smoothing reactor Ls where Ls represents the inductance thereof. This energy charges the capacitor of the filter F to an excessively high voltage. To suppress such excessive voltage a DC interrupting apparatus as shown in FIG. 3 has been developed in which R represents a resistor for charging capacitor C, Rs a nonlinear resistance element comprising silicon carbide which is used as the characteristic element of an arrestor, and TR.sub.1, TR.sub.2 and TR.sub.3 trigger gaps, that is, spark gaps provided with trigger electrodes as will be described in detail below. Concurrently, with the opening of the circuit interruptor CB, trigger gap TG.sub.1 is caused to discharge for passing the oscillatory discharge current Io through the arc of the circuit interruptor CB to interrupt the current I. The excessive voltage described above triggers the trigger gap TG.sub.2 whereby the energy stored in the smoothing reactor is dissipated by the nonlinear resistance element R.sub.s and the excessive voltage is suppressed. Thereafter, DC current corresponding to the voltage of the DC transmission line flows through the trigger gap TG.sub.2 and the nonlinear resistance element Rs. When the trigger gap TG.sub.3 is triggered after suppression of the excessive voltage, oscillatory discharge current I.sub. 1 of capacitor C flows through both trigger gaps TG.sub.2 and TG.sub.3 thus interrupting the current flowing through trigger gap TG.sub.2. As the insulating strength of the trigger gap TG.sub.2 recovers, the nonlinear resistance element Rs becomes isolated from the transmission line.
Another DC circuit interrupting apparatus as shown in FIG. 4 has also been developed in which a series circuit including a nonlinear resistance element Rs and a circuit breaker CB.sub.1 is connected across a DC circuit interruptor CB. When the circuit interruptor CB is opened a high recovering voltage appears across the separated contacts of the circuit interruptor so that the current I to be interrupted is transferred to the nonlinear resistance element R.sub.s. The current I.sub.1 flowing through the resistance element R.sub.s is smaller than the current I to be interrupted. Thereafter circuit breaker CB.sub.1 is opened to interrupt current I.sub.1. At this time since current I.sub.1 is limited by the resistance element R.sub.s, its interruption is easy. When the circuit interrupting apparatus shown in FIG. 4 is used in the DC transmission system shown in FIG. 2, the energy stored in the smoothing reactor LS is dissipated by the arcs of circuit interruptors CB and CB.sub.1 and the resistance element 4.
DC current interrupting apparatus are generally connected at both ends of a DC transmission line as shown in FIG. 5 in which DCCB.sub.1 and DCCB.sub.2 are DC circuit interrupting apparatus, LS.sub.1 and LS.sub.2 smoothing reactors, F.sub.1 and F.sub.2 filters, Re.sub.1 a rectifier (or inverter) and Re.sub.2 an inverter (or rectifier). Where a DC interrupting apparatus as shown in FIG. 3 is used, and where a fault occurs on the transmission line, the excessive voltage caused by the smoothing reactor LS.sub.1 would be suppressed by the nonlinear resistance element R.sub.s associated therewith but the excessive voltage caused by the smoothing reactor LS.sub.2 on the opposite end could not be suppressed because circuit interruptor DCCB.sub.2 would not be opened. On the other hand, when the circuit interrupting apparatus as shown in FIG. 4 is used in a DC transmission line, the energy of both smoothing reactors LS.sub.1 and LS.sub.2 could be adsorbed. With the construction shown in FIG. 4, however, as it is necessary to use additional circuit breaker CB.sub.2 for interrupting the current flowing through the resistance element the construction becomes complicated and expensive.